Electric Machine Amplifier with a Controlled Semiconductor Commutator
Abstract Generated abstract
This article presents an electromechanical adjustable-frequency amplifier that places a controlled semiconductor switch at the input of an asynchronous generator rather than at the output. The proposed arrangement uses semiconductor switching of the excitation winding to create a rotating magnetic field, allowing output frequency regulation while reducing the required rating of semiconductor power elements and improving reliability relative to static converters. The authors discuss operating conditions, suppression of self-excitation, rotor designs for widening the slip and frequency range, and recovery of slip energy to improve efficiency. A prototype rated at 800 W and operating from 30 to 170 Hz is reported, supporting the proposed system’s suitability for automated electric drives and high-accuracy frequency stabilization with variable-speed prime movers.
Full Text
Electromechanical Amplifier with a Controlled Semiconductor Switch
Sh. S. Roiz, B. E. Trofimenko, A. I. Skorospeshkin, M. L. Kostylev,
E. F. Obergán
(Recommended by the scientific seminar of the Departments of Electrical Machines
and General Electrical Engineering)
Various electromechanical, semiconductor, and machine-semiconductor converters are used as sources of alternating current of adjustable frequency.
Electromechanical converters of adjustable frequency have found limited application because of their inherent shortcomings: large weight and overall dimensions, difficult commutation conditions limiting the frequency range, low efficiency, and considerable inertia.
Since the beginning of 1960, static semiconductor converters using thyristors, as well as machine-semiconductor units, have been used to obtain alternating current of adjustable frequency. The use of the latter, especially in combination with contactless generators, is very promising for obtaining a stabilized frequency of high accuracy with variable shaft rotation speed [1].
The principal shortcoming of static semiconductor converters, as well as of machine-semiconductor systems with a controlled semiconductor switch (CSS) at the output, is that the power semiconductor elements are rated for the full load power, which substantially complicates and increases the cost of the installation. In addition, direct connection of semiconductor devices to the load reduces reliability: commutation failures of the valves are possible when the load changes, as is their failure under current overloads, overvoltages, etc.
Obtaining a good waveform of the current and voltage curves at the output over a sufficiently wide frequency range is also difficult.
These shortcomings are largely eliminated in a machine-semiconductor system when the CSS is placed at the input. Such a system operates as an amplifier of adjustable frequency. A description of the operation of the proposed adjustable-frequency amplifier (EMU-R4) is the subject of the present article.
The proposed electromechanical adjustable-frequency amplifier (EMU-RCh) consists of an asynchronous generator (windings \(W_2\) and \(W_3\), Fig. 1), excited from a direct-current source through the CSS and winding \(W_1\). The rotor of the asynchronous generator is rotated by a driving motor (for example, an asynchronous motor).
The excitation winding is made in the form of the armature winding of direct-current machines, the taps from which are connected not to the commutator plates, but to the cells of the CSS. The control winding may be either closed or open; however, from the standpoint of better utiliza-
Fig. 1. Schematic diagram of the EMU-RCh
of the installed power of the UPK, a multiphase split winding is advisable. The UPK, by ensuring a regular sequence of connection and disconnection of the phases of the control winding to the constant control source, creates in the machine a rotating magnetic excitation flux. The excitation flux induces an emf in the output winding \(W_2\) and in the rotor winding \(W_3\).
By changing the switching frequency of the phases of the control winding, it is possible to regulate the frequency at the output of the amplifier practically from several hertz to the maximum frequency determined by the number of poles of the asynchronous generator \(P_1\) and by the rotor speed \(n\).
\[ f_{\text{out max}} = P_1 \cdot n . \tag{1} \]
To ensure generator operation, the rotor of the asynchronous generator must rotate in the same direction as the excitation field, and the rotor speed must be greater.
Operating with negative slip, the generator delivers the active power supplied from the shaft to the load and to the excitation winding \(W_1\), thereby sharply reducing the control power. At a certain value of slip, load resistance, and active resistance in the rotor circuit, self-excitation may occur.
In order to ensure the amplifying properties of the generator over a wide range of load variation, it is proposed to provide it with a bias winding \(W_6\), wound around the stator yoke and connected in series in the control circuit. In this case, as the amplification factor increases, the current of windings \(W_1\) and \(W_6\) increases, the generator yoke becomes saturated, and self-excitation does not arise.
The asynchronous generator amplifies only the fundamental harmonic of the voltage. Owing to this, and also by applying the appropriate shortening of the windings and connection of the phases, it is possible, without special filters, to ensure a good waveform of voltage and current at the output over the entire frequency range of operation of the amplifier.
It is known that, in order to obtain maximum power from an asynchronous generator, it is necessary, as its slip increases, to increase the active resistance of the rotor circuit. Therefore the slip range of the generator, and consequently also the operating frequency range of the amplifier in which the generator provides a sufficiently large amplification (not less than 10–20), depends substantially on the rotor design.
The use of an ordinary squirrel-cage rotor gives a very narrow frequency range, \(1:(1.1—1.15)\). By using a rotor with deep slots, a Schenfer rotor, or a phase-wound rotor with external induction resistances of the Rozov design, the operating range can be increased to
$1 : (2—5)$ [2]. In these designs, the increase in rotor resistance with increasing slip is provided by the phenomenon of current displacement.
The main drawback of the indicated designs is that the slip losses are released in the rotor circuit and go into heating the machine, which reduces the efficiency at low frequencies and leads to an increase in the overall dimensions of the machine.
To eliminate this drawback it is proposed to make the rotor phase-type and to feed the slip power from the rotor through slip rings (1, 2, 3) to the input of the UПK, which increases the gain of the EMU-RCh and raises the efficiency of the unit.
In the contactless version, an additional stage is used for this purpose (windings $W_4$ and $W_5$); the stator winding $W_5$ is connected through rectifier B to the input of the UПK.
The number of pole pairs of the additional stage $P_2$ differs from the number of pole pairs of generator $P_1$, which makes it possible to combine them in one magnetic circuit.
If the field of winding $W_4$ rotates in the same direction as the rotor and the additional stage operates in the motor mode, then part of the generator slip energy is returned to the shaft, and part of the energy is supplied to the input of the UПK.
If the field of winding $W_4$ rotates opposite to the rotor and the additional stage operates in the mode of an asynchronous frequency converter, the slip energy, amplified by the additional stage, is supplied to the input of the UПK.
According to the circuit (Fig. 1), a prototype was manufactured and tested (Fig. 2) for a power of 800 W and a frequency range of 30–170 Hz [4].
Fig. 2. General view of the EMU-RCh with UПK
Thus, the proposed EMU-RCh with a UПK at the input, possessing high speed of response, flexibility of frequency and voltage regulation, a good waveform of the output voltage and current, and reliability, meets modern requirements and can be used in automated electric drives.
At the same time, owing to the amplifying properties of the asynchronous generator, the installed power of the power elements of the controlled semiconductor switch is substantially reduced in comparison with static converters of regulated frequency, and their operational reliability is increased.
The proposed circuit of an EMA–RF with a controlled semiconductor switch can be used to obtain a stabilized frequency of high accuracy when there is a prime mover rotating at variable speed.
REFERENCES
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K. E. Levitin, A. D. Melamed. Conversion and Stabilization of Frequency. TsINTI, 1962.
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M. M. Boychuk, Yu. V. Kostenko. Contactless Circuits for Automatic Motor Control. Kiev, 1967.
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E. F. Obergan, A. I. Skorospeshkin, M. L. Kostyrev, Sh. S. Roiz. Contactless electrical-machine amplifier of regulated frequency. Application 124503/24-7.
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Sh. S. Roiz, B. E. Trofimenko, A. I. Skorospeshkin. Experimental study of the main characteristics of an electrical-machine amplifier with a controlled semiconductor switch. Izv. TPI, Vol. 212, 1969.